Barry's only had one single mark in his whole life. It looks a lot like a small pencil and back when it first appeared, he didn't have any clue why exactly this would depict Iris. Of course he knew it's Iris, he is not stupid. He's always liked her a lot and once he actually got to live with her, spend more time with her, see her in every possible situation, it was only a matter of time until he really properly fell in love with her. As they grew up, he understood why exactly the pencil would describe her best, she's a reporter like you imagine one, always on and about to find out what is going on in the world and to tell people what they need to know.
He's lucky the brown mark is on his thigh, he's able to hide it from her, from Joe, from everyone because there's no way Iris would ever look at him as any different than her best friend. He can't tell her about his feelings, their friendship is too precious to break about something like that.
He didn't know those marks could fade, but when he thinks about it, it only makes sense. After all, you don't necessarily spend your whole life being in love with the same person. Sometimes feelings fade, and so do these marks.
He's glad it happens, he doesn't only notice because of the mark but also his reactions to just anything that Iris does.
She's with Eddie now and while he didn't trust him at first, the more Barry got to know him, the more he grew to like him. Eddie is a great guy, actually, he's nice and knows just when to make the right comments in different situations to ease the atmosphere, Barry can tell he cares about Iris a lot and he can accept that she's found somebody, that she's happy with someone who isn't him.
Apparently acceptance helps you to get over someone.
He even starts hanging out with Eddie when Iris is not around. He doesn't know when it actually first happened, who offered meeting up to whom but soon enough, they meet for coffee, for boxing, for movies, just everything you do with friends.
It's cool how Barry suddenly has actual friends to be around, his whole childhood Iris has been the only one who'd wanted to be near him. He can't blame the other children, apart from the fact that they were told he's a murderer's son, he'd also developed quite a social awkwardness. But now he's got Cisco and Caitlin, too, and strangely enough, Eddie, who is just so different from him but at the same time they manage to get along and have fun together.
He should've realized it way sooner, the signs have been there but he's an idiot. He's noticed how his heart warms up at the sight of Eddie's smile whenever they see each other, he has caught himself looking into his eyes for longer than necessary, he couldn't stop himself from holding on to a hug for several moments. It only really hits him, though, when he looks into the mirror one morning to see something on his neck, open to display, a place he won't be able to hide this easily.
It's a thin line with an open triangle on the lower end of it, a sign Barry has never seen before, he doesn't know the literal meaning of it, but he's pretty sure about the meaning it has for him: at some point he fell in love with Eddie Thawne.
Eddie has lot of those marks that appear whenever you fall in love with someone. It's no secret, he falls in love quite easily and often, the longest it's taken him was in college when he's dated a guy who didn't even look for something too serious. He fell in love anyway, even if it's been after a few months. A few times these marks appeared on mere sight of someone, he'd notice them at the end of the day and wouldn't even be able to tell whom exactly they're supposed to show. They've already started to fade by then.
They're all faded by now except for the one on the back of his left hand. It kind of looks like a fish and though he has no idea why a fish would symbolize Iris, he knows it's hers because it has appeared shortly after they started dating, when he first felt a rush of warmth spread out on his face at the sight of her.
She knows of all his other marks, she even knows the stories to most of those he remembers (when they were worth remembering), she knows how he falls in and out of love easily. She also knows her mark on his body hasn't faded yet, it's as dark as ever.
His mark on her body is something that looks like a small M, it shines brightly against the brown of her skin, and it hasn't gotten any less either.
It's quite fascinating to have those signs of love on your body, to watch them appear and try to figure out what they mean, but to know for yourself whom exactly they mean. They appear on random places on your body and sometimes, if you don't particularly check for them, some go unnoticed.
If Eddie had lifted his arm one morning when he's in the bathroom, still shirtless after a shower, a look into the mirror would have told him about a small lightning bold that appeared on the inside of his upper arm only a year after he and Iris started dating, as dark as the mark that symbolizes her love.